The present invention relates in general to an automatic weaving loom of the shuttleless type and particularly to a shuttleless weaving loom having a weft inserting arrangement in which a measured length of weft yarn is cyclically picked into a shed of warp yarns by means of a jet stream of fluid which is shot from a nozzle into the shed of the warp yarns in predetermined cycles. More specifically, the present invention is concerned with a device for producing such a jet stream of fluid in the weft inserting arrangement of the specific nature.
As is well known in the art of shuttleless weaving looms, the behaviour of the weft yarn picked into the shed of warp yarns by means of a jet stream of fluid varies markedly depending upon the fluid pressure developed in the nozzle from which the weft yarn is to be shot and the duration of the cycle during which the fluid is discharged from the nozzle. It is, for this reason, desirable to have these parameters varied in consideration of the nature of the weft yarns used and the width of the woven fabric to be produced. If, for example, a spun yarn is used as the weft yarn and water is used as the medium to entrain the pick of the weft yarn, it is required that the pressure imparted to the stream of water to be shot from the nozzle be continuously varied as the time lapses during each picking cycle because the behaviour of the picks of spun yarns in general is more seriously affected by the stream of water than those of other types of yarns such as, for example, filament yarns and should be entrained on a greater volume of water stream than the water streams used to pick the filament yarns.
If the pressure imparted to the fluid to be discharged from the nozzle is maintained constant throughout each of the picking cycles, the pick of the weft yarn shot from the vicinity of one end of the shed of the warp yarns will be gradually decelerated as the forward end of the weft yarn approaches the opposite end of the shed by reason of the resistance of air to the jet stream of water entraining the weft yarn. The velocity of the weft yarn end reaching the opposite end of the shed of the warp yarns thus becomes lower by several per cent than the initial velocity of the weft yarn shot from the nozzle and, accordingly, the pick of the weft yarn being inserted through the shed of the warp yarns is higher in velocity in its portion close to one end of the shed than its portion close to the opposite end of the shed. The weft yarn therefore tends to be slackened when received by the warp yarns forming the shed so that not only irregularity is created in the tension of the weft yarn forming a woven fabric but it happens that the weft yarn has a loop formed in its intermediate portion, giving rise to deterioration of the commercial value of the fabric produced. It is, for this reason, desirable in shooting the jet steam of fluid from the nozzle to have the velocity of the jet stream continuously diminished during each of the weft picking cycles so that the pick of the weft yarn is propelled at a lower velocity toward the end of each picking cycle than at an initial stage of the picking cycle. The velocity of the jet stream of fluid ejected from the nozzle is substantially in direct proportion to the pressure imparted to the fluid which is about to be discharged from the nozzle if the nozzle has a fixed area at its outlet end. The velocity of the jet stream of fluid from the nozzle may therefore be controlled by gradually reducing the fluid pressure to be developed in the nozzle. To achieve this end, it will be necessary to impart a higher pressure than usual to the jet stream of fluid to be discharged from the nozzle at an incipient stage of the picking cycle so that the weft yarn inserted through the shed of warp yarns is sufficiently taut throughout its length covering the entire width of the shed in each picking cycle.